A well-known method for obtaining subsurface structural information involves placing a sound source and a series of sensors in a body of water. A sound signal is generated. The sound wave travels down to the sea floor and beyond and is reflected from the sea floor surface and from other material interfaces below the sea floor surface. The reflected sound signal returns and is detected by the sensors. The system of source and sensors is moved along the survey profile and the emission of sound pulses is repeated at regular intervals as described above. As the system progresses, each subsurface reflection point (called a common mid-point or CMP) will be repeatedly sampled from many pairs of source and sensor positions. The sensed signals are processed to obtain information relating to the structure of the sub surface geology. More particularly, the sensors are placed at a longitudinal row along the length of a so-called streamer, which is towed by a vessel. The vessel may in fact tow multiple streamers arranged in parallel, to provide a moving 2D grid of sensors to collect 3D data.
A problem associated with this art is “ghosting”. When the sound pulse is emitted, an energy wave front radiates from the source. Some energy will travel into the ground and return as useful primary reflection data. Some energy will be reflected from the sea surface and then travel into the ground. This energy will be time delayed and reverse polarity compared to the primary energy and is called the “source ghost”. The same effect will be observed at the sensors with primary reflection data arriving first followed by the “receiver ghost” energy reflected down from the sea surface above.
The primary and ghost reflection data will both constructively and destructively interfere with each other. As a result of destructive interference, the sensor output signal has a frequency spectrum which contains a notch at certain frequencies which depend on the depth of the corresponding sensor or source below the water surface. The notch frequency fN is generally given by the expression fN=c/2D, in which c indicates the speed of sound in the medium and D indicates the depth of the sensor or source.
This ghosting problem as such is known, and measures have already been proposed to overcome this problem. One previously proposed solution is to use a slanted streamer so that individual sensors are located at mutually different depths. The ghosting phenomenon will then result in each sensor having different spectral characteristics. When data from the sensors is eventually summed, a uniform output spectrum will result because the location of the spectral notches will vary for each sensor contributing to the summation.
By way of a further example where a slanted streamer is deployed, reference is made to US patent application US-2013/0135966-A1. In the method and system disclosed in this document, two reflection measurements with the same separation between source and receiver (called the “offset”) are performed at the same CMP position. One measurement is made with the source at one side of a sensor (“upstream”). The system then moves along the survey profile until the source is in the same position as the sensor in the first measurement. Another measurement from the same CMP is then made with the source at the opposite side of a second sensor (“downstream”). Because the streamer is slanted, the depth of the second sensor will be different from the depth of the first sensor with correspondingly different spectral notches. The sensor signals resulting from these two measurements are de-ghosted during processing using established techniques.